Many OFDM-based systems, such as the Universal Mobile Telecommunications Standard Long Term Evolution (UMTS LTE), require that receivers of the system are able to process a received OFDM-signal to acquire a relatively high signal-to-noise ratio (SNR). The receiver should be able to process the signal without introducing impairments or noise (or at least without introducing impairments or noise that are of a severe nature). Further, the receiver should be able to adjust for impairments and/or noise introduced by the channel over which the received OFDM-signal was transmitted.
Furthermore, many OFDM-based systems employ complex transmission schemes, such as Multiple-Input Multiple-Output (MIMO) and/or large signal constellations, which may further increase the demands on the receiver.
To be able to meet such demands of high SNR in receivers operating in OFDM-systems, an expensive radio design may be required. Alternatively (or additionally) methods may be employed that are able to handle radio imperfections resulting from a non-optimal radio design.
One radio imperfection is IQ-imbalance. IQ-imbalance is one of the more limiting radio imperfections, and is thus important to dispose of or at least suppress.
IQ-imbalance may be generated by anything that affects the in-phase (I) and quadrature (Q) components of the received OFDM-signal differently. One example source of IQ-imbalance is a local oscillator of a receiver or a transmitter (or both). Another example source of IQ-imbalance is mismatch between one or more blocks in the respective I- and Q-paths of the receiver chain. Examples of blocks that may experience such mismatch are amplifiers and channel filters.
IQ-imbalance can be modeled, in the receiver, as a difference in phase and amplitude between the in-phase and quadrature oscillator components (i.e. the carriers). After down-converting the received signal to a baseband signal in down-conversion mixers, this difference in phase and amplitude results in a leakage between the in-phase and quadrature components of the baseband signal. Thus, the real part of the symbols will affect the imaginary part of the symbols, and vice versa.
In OFDM, data is transmitted in parallel on a number of sub-carriers (or sub-carrier frequencies), which may be efficiently implemented by using an inverse fast Fourier transform (IFFT) in the transmitter, and a fast Fourier transform (FFT) in the receiver. If the size of the FFT is N, then N samples at the output of the FFT are referred to as an OFDM-symbol (i.e. a frequency domain OFDM-symbol).
Each OFDM-symbol thus comprises data on N sub-carriers. Each such piece of data will be referred to as a symbol (in contrast to an OFDM-symbol), and may comprise a pilot symbol or an information symbol. In UMTS LTE, a symbol as described above may be denoted a resource element, and a pilot symbol may be denoted a reference signal.
In an OFDM-based system, the baseband signal is thus transformed, in the receiver, to a frequency domain signal and this is commonly achieved by applying an FFT to the baseband signal. When transformed to the frequency domain, the IQ-imbalance affects the frequency domain signal in frequency pairs. Thus, the symbols on sub-carrier N−k leak into sub-carrier k and vice versa. This may be expressed by the following frequency domain expression:YIQ(k)=K1Y0(k)+K2Y0*(N−k),  (1)where * denotes conjugate, K1 and K2 are factors that depend on the phase and amplitude mismatch (for example in the local oscillator or of blocks in the respective I- and Q-paths of the receiver chain), Y0(k) is what the received signal would have been if there was no IQ-imbalance, and YIQ(k) is the actually received signal. It may be noted that the notation of sub-carrier N−k is equivalent to sub-carrier −k. This is due to the N-periodicity of the FFT. Throughout this application, sub-carrier N−k will be denoted the mirror sub-carrier of sub-carrier k, and sub-carriers k and N−k will be denoted a frequency pair.
The leakage from a sub-carrier to another sub-carrier is a form of inter-carrier interference (ICI), and will degrade the SNR in the receiver. Thus, in order to achieve a high SNR while allowing for a less expensive radio design, it may be desirable to measure (or estimate) the IQ-imbalance and perform compensation on the received signal for the estimated IQ-imbalance. For example, the value ρ=K2/K1* can be estimated. The estimated value {circumflex over (ρ)} can then be used to perform compensation on the received signal. The estimated value {circumflex over (ρ)} may, for example, be determined based on known pilot values and known channel values (e.g. channel estimates). If the value {circumflex over (ρ)} is accurately estimated, the compensation will cancel the leakage from the mirror sub-carrier completely.
Estimating the value ρ accurately requires knowledge of the channel as well as of the transmitted symbols for each frequency pair used in the estimation. Channel information could, for example, be obtained from the channel estimator. Knowledge of transmitted symbols is commonly obtained by the use of pilot symbols. Thus, to be able to perform an accurate estimation of the value ρ pilots need to be distributed on both sub-carriers of each frequency pair used in the estimation.
However, pilot symbol distribution is in general defined in the standard to be applied. Thus, access to pilot information on frequency pairs depends on how the pilot distribution is defined in the standard. In UMTS LTE, for example, the pilots are not placed on mirror frequency pairs. This is a severe obstacle when estimating the value ρ and performing IQ-imbalance compensation, and limits the possibility of straightforward use of this approach.
Thus, there is a need for improved methods of and arrangements for estimating IQ-imbalance of a received OFDM-signal.